1. Field of Invention
This invention relates generally, to spectroscopic based optical fiber sensors. Particularly, this invention relates to absorption, fluorescent, phosphorescent and chemiluminescent based sensors.
2. Description of Prior Art
Spectroscopic based optical fiber sensors are used throughout numerous industries for the detection of temperature and various chemical species comprising a liquid or gas. These sensors have been developed using, primarily, two separate approaches: the optrode (or optode) and the distributed sensing approach.
Optrodes are the simplest type of optical fiber sensors. Peterson et al, U.S. Pat. No. 4,200,110, discloses an indicator at the distal end of the fiber that is excited by a light source located in the proximal end. The excitation light travels through the fiber and interacts with the indicator producing a spectral signal (fluorescence, phosphorescence, chemiluminescence and/or absorption). The signal travels back to the proximal end, is collected by a detector and is correlated with the parameter that is being measured. In this case, the fiber, having no sensitive regions along its length to produce a change in the signal, serves only as a conduit of the light, which propagates undisturbed from the proximal fiber end to the indicator and back. Each point along the fiber sensor requires a separate fiber optically communicating between the light source and the indicator, potentially creating a complex system of several of fibers.
In the distributed sensing approach, the entire fiber or sections of the fiber, act as a sensor. In one case, the fiber is manufactured with a single cladding sensitive to the parameter being measured. In another case, several cladding sections are removed exposing the fiber core. Next, the bare core regions are coated with a reactive agent, often having an index of refraction similar to that of the cladding. In either approach, these reactant regions can be probed by an excitation light. Not only does the fiber act as a conduit for the signal, the fiber itself is sensitive, resulting in a multipoint, quasi distributed, sensing device. Whereas, the optrode approach requires several strands of optical fibers to make multiple spatial measurements, the distributed sensing approach usually requires just a single optical fiber strand. Therefore, the advantage of distributed sensing is that it can make multiple spatial measurements with a single device.
Within the distributed sensing approach, there are two primary methods for probing to the sensitive regions of the fiber, axial excitation and transverse excitation, transverse excitation being judged to be a superior technique by the present invention.
Axial excitation is commonly used as a means for probing the sensitive cladding. In axial excitation, light that is injected from one end of the fiber, along the axis, interacts with the surrounding cladding via its evanescent wave tail. The cladding absorbs the excitation light in the evanescent region producing either an absorption or luminescent signal that can be detected at the end of the fiber.
The axial excitation technique, however, has various inherent drawbacks. The interaction between the evanescent tails of the excitation light with the sensitive cladding is very small requiring a high power source, an expensive detection scheme and/or a very long optical fiber. Additionally, depending on the arrangement, the collinear alignment of the light source (such as a laser) with the axis of the optical fiber can be challenging, possibly requiring careful handling and calibration.
Schwabacher, international publication number WO 01/71316 ('316), demonstrates a linear array of chemosensors arranged along an optical fiber, each reactant region in the array being sensitive to a chemical species. Each successive reactant region is separated by a substantially inert region, such as cladding. This substantially inert region must have a minimum length, the preferable length being stated as 250 cm. Publication '316 demonstrates both the axial and transverse methods of excitation, axial being the preferred mode.
In the preferred embodiment, '316 employs a narrow axial laser pulse to introduce an excitation light to the optical fiber. Each reactant region is separated by a minimum distance along the fiber, the region between the reactive regions being substantially inert. This relative long inert section is required by the technology utilized by '316, to prevent overlap of fluorescent traces from successive reactant regions. An excitation light from a source (such as a laser, diode laser, gas laser, dye laser, solid state laser, LED, etc) is introduced axially to an optical fiber, the light then being delivered to the reactant regions.
In order to determine which reactant region, among several or even hundreds, is producing a signal, the time delay between the excitation pulse and return signal must be precisely known and correlated with the distance each particular reactant region is from the source, measuring time, distance, and wavelength by use of precise instruments such as the oscilloscope and photomultiplier tube. This arrangement requires an extremely long length of fiber in order to measure hundreds of species, increasing the overall size and complexity of the analyzing device. Furthermore, the precision instruments can increase the overall cost of the instrument significantly.
The excitation light can also be introduced to the reactant regions on the sensing fiber by an excitation fiber or fibers. This also requires the axial introduction of light to the excitation fiber. One excitation fiber per reactant region is required in one embodiment, each fiber introducing the excitation light transversely to the reactant region of the sensing fiber.
Another embodiment requires the use of beam splitters to deliver the excitation light transversely to the reactant regions. The beam splitting technique make use of expensive high power lasers wherein the intensities decay as more beam splitters divert the excitation light to the sensitive coating.
In another scheme, the excitation fiber is prepared by removing its cladding from small sections along its length, these sections then being installed adjacent to the reactant regions on a nearby sensing fiber, allowing its evanescent field to transversely excite the sensing fiber. A disadvantage is that the evanescent field of the excitation fiber is very weak delivering very little power to the sensing fiber. Additionally, other methods of axial and transverse excitation are revealed; however, these methods were, on average, not cost effective.
Although it is acknowledged that these embodiments of '316 are operational, they are limited by complexity, manufacturing expense, and robustness of design. In order to manufacture alternating sections of reactant and inert regions, cladding must be removed only in the reactant regions, leaving it intact in the inert regions. This alternating removal of cladding increases the expense and complexity of mass production, limiting automation options in manufacture.
Additionally, other techniques utilized in industry require the use of expensive instrumentation such as an optical time domain reflectometer (OTDR). Costing on the order of U.S. $20,000 or more, the OTDR adds considerable expense to any system that uses the axial excitation technique. Also, the wavelengths availability of the OTDR systems are limited, restricting the choices of reagents that can be used with the sensor. A further disadvantage of present systems is interference of the signal detected by the OTDR caused by inadvertent bends and physical irregularities in the waveguide material, varying the fiber's refractive index. Furthermore, present techniques lack refinement of spatial resolution, on the order of approximately 10 cm. A more refined spatial resolution is needed.
Again, it is acknowledged by this inventor that transverse excitation of the sensitive region is a superior technique, producing a substantial quantity of fluorescent signal. However, past inventors failed to identify that side excitation, when properly done, can probe very small sections of a sensitive fiber leading to a sensor with a very high spatial resolution. High spatial resolution, less than 5 mm, is desired in applications wherein there is a strong variation of the temperature and/or concentration of a chemical species along the length of the optical fiber. The monitoring of chloride ions in concrete structures, serves as an example where the sensing can be made at discrete narrow locations along the fiber. Previous endeavors also failed to provide a simpler excitation technique that leads to a low cost and rugged sensor.
What is needed is an inexpensive probing light source that can additionally provide a high spatial resolution to the fiber sensor, on the order of 5 mm or less, enabling the pinpointing of the exact location of detection. What is needed, additionally, is a cost effective optical fiber sensor system that uses inexpensive, off the shelf, commercially available devices that can be fabricated by automated means. What is also needed is a flexible device that can be used throughout the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Additionally, what is needed is a rugged sensing device that can be easily aligned and is not affected by outside interference such as bending and ambient light. In addition, a generic design that can be adapted to monitor different chemical species is needed. What is also needed is an intense, and yet, cost effective probing light source for a fluorescent based and absorption based fiber that can produce a strong signal that can be easily detected. And what is finally needed, is a modular sensing system design that can be easily updated with the evolving technology.